This invention relates to capacitors and their production, and more particularly to capacitors having amorphous hydrogenated carbon dielectrics with high dielectric strength and low dissipation factor.
Capacitors are commonly employed as elements in electrical circuits to reduce voltage fluctuations in electronic power supplies, to transmit pulse signals, to generate or detect electromagnetic oscillations at radio frequencies, to provide electronic time delays and the like. Typical capacitors have two or more conductive layers separated by at least one dielectric layer.
Various materials are known to be useful as dielectric layers in capacitors. In recent years, a material of this type which has been of increasing interest is amorphous hydrogenated carbon, also known as diamondlike carbon and hereinafter sometimes designated "DLC". It is described in U.S. Pat. No. 5,275,967 as being principally composed of carbon of SP.sup.3 structure, having a diamond structure over a short distance but showing disordered amorphous character over a longer distance. As described in Chemical and Engineering News, May 15, 1989, pp. 32-33, films of this material are generally amorphous but can have very small crystals, typically less than 2-20 nm. in diameter, and can contain from almost 0 to more than 50% hydrogen. When they are characterized by greater than 0.2 gram-atoms of carbon per cc., they can possess properties very close to diamond, including high dielectric strength and low dissipation factor.
It is of interest to raise the dielectric strength and lower the dissipation factor of DLC even further. Higher dielectric strength would allow a corresponding increase in operating electric stress and a decrease in the film thickness required to achieve a specified voltage rating. A decrease in film thickness permits a decrease in deposition time, which in turn reduces cost. It also increases capacitance per layer, whereupon fewer dielectric layers are necessary for a given total capacitance. Decrease in dissipation factor increases capacitor life and provides higher circuit efficiency.
Dissipation factor generally varies with the frequency of the voltage signal at the capacitor terminals. A typical capacitor having DLC dielectric layers will have an adequately low dissipation factor at low to moderate frequencies, on the order of 60-100,000 Hz, which are characteristic, for example, of motors. At frequencies above 1 MHz, however, the dissipation factor rises substantially and may render the capacitor incapable of use in articles operating at such frequencies.
The present invention is based on the discovery that the doping of DLC films with low proportions of silicon results in a substantial increase in dielectric strength and decrease in dissipation factor at high frequencies. It is known that doping with silicon decreases frictional coefficient and improves tribological properties; Demichelis et al., Materials Science and Engineering, B11,313-316 (1992). However, the improvement of dielectric properties by silicon doping is not believed to have been previously reported.